उत्सीदेयुरिमे लोका न कुर्यां कर्म चेदहम्। सङ्करस्य च कर्ता स्यामुपहन्यामिमाः प्रजाः ॥

utsīdeyur ime lokā na kuryāṁ karma ched aham sankarasya cha kartā syām upahanyām imāḥ prajāḥ

These worlds would fall into ruin, if I did not perform action; I should be the author of confusion of castes, and should destroy these creatures.

In plain words: Krishna says the worlds would fall apart if he stopped acting. He would cause social disorder, including the mixing up of castes (the traditional social groups), and he would bring harm to all beings.